The process of dyeing and curing leather is an ancient one, with roots at least as far back as ancient Assyria. What’s most surprising is how little of the process has changed; despite being mechanized over time, the steps to curing leather are much the same as they were hundreds if not thousands of years ago. It’s through these practices that we keep our leather so consistently beautiful at Quero Handmade Shoes.
Natural leather most often comes from cowhide, sourced from cows that were slaughtered for their beef. The hide is then removed carefully by skilled workers; a slip of the hand can drastically decrease the value of the hide.
Sometimes, the hides are stacked, with layers of pure salt between them. This prevents any microorganisms from attacking the leather until it is ready to be worked. In some places, the hides are chilled instead, and cleaned more thoroughly by machine before being stored in brine, a mixture of salt and water. The hides have to cure for at least a full day before the next step, but sometimes they are stored this way far longer; it all depends on the need for leather and the pace of the factory where they were preserved.
Next, the hides must be rinsed in a large drum or vat in order to remove the salt that has soaked into the material. Then lime is added to the drum to remove the hair from the hide. The liming process also begins to change the structure of the principal protein in the hide, collagen, breaking it into smaller pieces. This ensures that the final product will be more supple and easy to work.
Bating is next. This process adds acid salts such as ammonium chloride and proteases – enzymes that break down proteins – to the surface of the leather in order to make it yet more supple and smooth, and to remove any remaining scud. Workers move the hide into a warm bath to activate the enzyme, and rinse it until no chemicals or unwanted detritus remains. Finally, more acid is added until the pH of the solution is very low. At this point, the hide should be very pliable with a relatively even surface, and is ready to be tanned.
Tannin is a caustic chemical initially found in plant matter. When the tannin interacts with the proteins in the hide, it causes the material to become stronger and more resistant to breakdown over time. Tannin used to be extracted from various plant materials such as oak and sumac bark, but these days tannin’s job is usually performed by chromium salts dissolved in water. The hides are soaked for eight to twelve hours in the tanning solution before they are finished.
Depending on the desired result, sometimes dyes are added straight into the chromium solution. Other times, the leather is dyed after the tanning process is complete. Either way, the chromium atoms that remain in the leather are the most likely spot for the dye to bind – the chemical that tanned the leather also helps to ensure that the dye is taken up readily and evenly!
Finally, the leather is ready to be worked. If the final product is to be finished suede, the leather will be buffed roughly. If the product will have a special finish, such as patent leather, this is when that finish is applied.
The name ‘patent leather’ comes from the fact that the leather was finished using a secret, patented process and formula. Many manufacturers had their own special blends to paint onto shoes to create patent leather, employing such exotic ingredients as copal, whale oil, and lamp black. Resinous combinations that retained enough flexibility to be used for shoes were so difficult to formulate that some manufacturers even lied on their patents, leaving out secret ingredients or fudging the proportions so that it was impossible for another manufacturer to steal their unique combination of oils, resins, and dyes.
These days, patent leather is typically spray-on polyurethane, or acrylic before being swiftly dried to decrease the chances of cracking or particles falling into the wet spray. A clear topcoat is sprayed on last, just like a nail polish. Ideally, the formula shouldn’t crack, and it should maintain its finish in the face of all but the roughest scuffs.
Then, the leather finally arrives at the doorstep of your shoemaker, ready to be shaped into something practical, beautiful, and fashioned perfectly for your foot.